The present invention relates to a process for the preparation of acrylic acid, a device for the preparation of acrylic acid, a process for the preparation of polyacrylates, a device for the preparation of polyacrylates, the use of acrylic acid, and acrylic acid, polyacrylates and chemical products containing these, in particular superabsorbers and diapers.
Acrylic acid is a starting compound of great technical importance. It serves inter alia for the preparation of polyacrylates, in particular crosslinked, partly neutralized polyacrylates which have a large capacity for absorption of water in the dry and substantially anhydrous state. This can make up more than ten times its own weight. Because of the high absorption capacity, absorbent polymers are suitable for incorporation into water-absorbing structures and objects, such as e.g. baby diapers, incontinence products or sanitary napkins These absorbent polymers are also called “superabsorbers” in the literature. In this connection, reference is made to “Modern Superabsorbent Polymer Technology”; F. L. Buchholz, A. T. Graham, Wiley-VCH, 1998.
Acrylic acid is conventionally obtained from propylene by a gas phase oxidation which proceeds in two stages, in which propylene is first oxidized to give acrolein, which is then reacted further to give acrylic acid. A disadvantage of this two-stage process for the preparation of acrylic acid is on the one hand that the temperatures used in the two reaction stages, which are conventionally between 300 and 450° C., lead to the formation of undesirable cracking products. This in turn results in an undesirably large amount of impurities being obtained, which may also be polymerizable and can be incorporated into the polymer backbone acid in the presence of crosslinking agents. This has an adverse effect on the properties of the superabsorbers. Aldehydes in particular, such as, for example, furfural, acrolein or benzaldehyde, furthermore act as inhibitors in the free radical polymerization, with the consequence that the polymers still contain considerable amounts of soluble constituents if they are not extracted from the acrylic acid employed for the polymerization by elaborate purification steps.
It is also to be noted that if superabsorbers are employed in hygiene articles and in products for wound treatment, the toxic acceptability requirement is very high. This means that the educts employed for the preparation of the superabsorbers likewise must have the highest possible purities. It is therefore of great importance to provide acrylic acid as the main educt in an inexpensive manner in a form which is as pure as possible for the preparation of superabsorbers.
A further disadvantage of the conventional process for the preparation of acrylic acid is that the educt employed (propylene) is prepared from crude oil and therefore from non-regenerating raw materials, which from economic aspects above all is a disadvantage in the long term above all in view of the increasingly more difficult and above all more expensive production of crude oil.
Here also, some approaches for counteracting this problem are already described in the prior art. It is thus known in particular to obtain acrylic acid starting from hydroxypropionic acids, for example from 2-hydroxypropionic acid or 3-hydroxypropionic acid, by dehydration of the hydroxypropionic acid.
The preparation of 2-hydroxypropionic acid by a fermentative route from biomass, such as glucose or molasses, and by a synthetic route is known inter alia from PEP Review 96-7 “Lactic acid by Fermentation” by Ronald Bray of June 1998.
WO-A-03/62173 describes the preparation of 3-hydroxypropionic acid, which can serve inter alia as a starting substance for acrylic acid synthesis. In this context, according to the teaching of WO-A-03/62173 α-alanine is first formed fermentatively from pyruvate, and is then converted into beta-alanine by means of the enzyme 2,3-aminomutase. The β-alanine in turn is converted via β-alanyl-CoA, acrylyl-CoA, 3-hydroxypropionyl-CoA or via malonic acid semialdehyde into 3-hydroxypropionic acid, from which acrylic acid is obtained after a dehydration.
WO-A-02/42418 describes a further route for the preparation of, for example, 3-hydroxypropionic acid from regenerating raw materials. In this context, pyruvate is first converted into lactate, from which lactyl-CoA is subsequently formed. The lactyl-CoA is then converted via acrylyl-CoA and 3-hydroxypropionyl-CoA into 3-hydroxypropionic acid. A further route for the preparation of 3-hydroxypropionic acid described in WO-A-02/42418 envisages the conversion of glucose via propionate, propionyl-CoA, acrylyl-CoA and 3-hydroxypropionyl-CoA. This publication also describes the conversion of pyruvate into 3-hydroxypropionic acid via acetyl-CoA and malonyl-CoA. The 3-hydroxypropionic acid obtained by the particular routes can be converted into acrylic acid by dehydration.
WO-A-01/16346 describes the fermentative preparation of 3-hydroxypropionic acid from glycerol, in which microorganism which express the dhaB gene from Klebsiella pneumoniae (a gene which codes for glycerol dehydratase) and a gene which codes for an aldehyde dehydrogenase are employed. 3-Hydroxypropionic acid is formed from glycerol via 3-hydroxypropionaldehyde in this manner, and can then be converted into acrylic acid by dehydration.
Regardless of the enzymatic route by which the hydroxypropionic acid is obtained by fermentation, after the fermentative process an aqueous composition is present which, in addition to the hydroxypropionic acid, still contains numerous by-products, such as, for example, cells, non-converted biomass, salts and metabolism products formed alongside the hydroxypropionic acid.
In order to use the hydroxypropionic acid as a starting material for the preparation of acrylic acid, it is advantageous first to concentrate and to purify this. In this context, in connection with the purification of 3-hydroxypropionic acid it is known from WO-A-02/090312 first to add ammonia to the fermentation solution for neutralization, in order to convert the 3-hydroxypropionic acid into its ammonium salt. The fermentation solution obtained in this way is subsequently brought into contact with a high-boiling organic extraction agent and the mixture is heated, ammonia and water being stripped off in vacuo and the free 3-hydroxypropionic acid formed being extracted into the organic phase. An organic phase containing 3-hydroxypropionic acid is obtained in this manner, from which the 3-hydroxypropionic acid can be re-extracted or in which, after addition of a suitable catalyst, the 3-hydroxypropionic acid can be converted into acrylic acid. The disadvantage of the “salt splitting” process described in WO-A-02/090312 is, inter alia, that on the one hand aqueous phase to be purified must contain the 3-hydroxypropionic acid in an amount of at least 25 wt. % in order to render possible an effective purification by the so-called “salt splitting process”. Since such high 3-hydroxypropionic acid concentrations cannot be achieved in the fermentation processes currently known for the preparation of 3-hydroxypropionic acid, it is necessary first to concentrate the 3-hydroxypropionic acid concentration in the fermentation solution. This is conventionally effected by evaporating the water out of the fermentation solution. Further disadvantages of the purification process described in WO-A-02/090312 are that on the one hand this process envisages the addition of a high-boiling organic solvent to the fermentation solution, which must be separated off from the 3-hydroxypropionic acid in the further course of the process, and that on the other hand in the purification of 3-hydroxypropionic acid from fermentation solutions which include numerous organic by-products, these organic by-products are also at least partly extracted into the organic solvent. In this case it is necessary for the organic phase obtained in the salt splitting to be purified further in order to obtain 3-hydroxypropionic acid with a satisfactory purity.
In connection with the purification of 2-hydroxypropionic acid (=lactic acid) from aqueous solutions, it is known from WO-A-95/024496 that the aqueous solution is first combined by means of an extraction agent comprising a water-immiscible trialkylamine having a total of at least 18 carbons in the presence of carbon dioxide under a partial pressure of at least 345×103 pascal to form an aqueous and an organic phase, and the lactic acid, which is in the organic phase, is subsequently extracted from the organic phase, for example by means of water. Here also the disadvantage of the purification process is that in the case of a fermentation solution as the starting composition, not only the lactic acid but also other by-products are dissolved in the organic phase, so that no pure lactic acid solution is obtained in the re-extraction with water.
In the dehydration of hydroxypropionic acid to give acrylic acid, water is furthermore obtained. Even if quite concentrated hydroxypropionic acid solutions or even relatively pure hydroxypropionic acid are employed, purification of an aqueous mixture of acrylic acid, hydroxypropionic acid and optional by-products obtained during the dehydration must therefore be carried out.
WO-A-2004/76398 proposes a vacuum distillation with dodecanol as an addition for separation of a mixture of acrylic acid and 3-hydroxypropionic acid. However, this gentle process is disadvantageous because of the use of dodecanol.
The present invention was based on the object of mitigating or even overcoming in context the disadvantages emerging from the prior art.
In particular, the present invention was based on the object of providing a process for the preparation of acrylic acid from a fluid phase, preferably based on an aqueous fermentation solution, with which an acrylic acid which is as pure as possible or an aqueous acrylic acid which is as pure as possible can be obtained under conditions which are as gentle as possible and simple. It should be possible for this purification process to be carried out with the lowest possible or even without the addition of organic solvents.
The present invention was furthermore based on the object of providing a process for the preparation of acrylic acid and a process for the preparation of polyacrylate, which renders possible a preparation of acrylic acid, in particular from biomass, which is as gentle and simple as possible.
The present invention was also based on the object of providing devices for the preparation of acrylic acid and polymers thereof with which such processes can be carried out.